Page 28 - Reliability and Maintainability of In service Pipelines

Introduction 17

Figure 1.1 Cross-section of a pipe in static equilibrium.

2 3 σ h w 5 P 3 2r ð1:1Þ
The above equation representing hoop tensile stress, ultimately results to:
Pr
σ h 5 ð1:2Þ
w
In order to determine pipe wall thickness, the formula is rewritten as:

PD 0
w 5 ð1:3Þ
2σ P
where

σ P 5 stress from internal pressure (psi)
P 5 pressure (psi)
D 0 5 steel cylinder outside diameter (inches)
w 5 thickness of steel pipe
The most common internal pressure that is analyzed for the design of pipelines
is operating pressure, known as the working pressure (P w ).
The allowable hoop tensile stress of the steel pipe is typically limited to a value
equal to 50% of the minimum yield strength of the pipe and is higher (75%) with
transient pressures (Whidden, 2009). As this pressure is usually referred to as tem-
porary due to its momentary spike, a greater hoop tensile stress is allowed.
Following this design procedure, the pipes are hydrostatically tested to ensure fac-
tors such as tensile stresses do not exceed yield strengths and capacity of the steel.
The minimum thickness for steel pipes often depends on the safety of handling
and installation. Typically, D=w ratios of up to 240 are used, however for pipes
constructed with special design specifications, D=w ratios of up to 288 and higher
can be used, which are generally seen in pipelines that are used in irrigation and
hydroelectric systems.